Increasingly, resources such as heavy crude oils, bitumen, tar sands, shale oils, and hydrocarbons derived from liquefying coal are being utilized as hydrocarbon sources due to decreasing availability of easily accessed light sweet crude oil reservoirs. These resources are disadvantaged relative to light sweet crude oils, containing significant amounts of heavy hydrocarbon fractions such as residue and asphaltenes, and often containing significant amounts of sulfur, nitrogen, metals, and/or naphthenic acids. The disadvantaged crudes typically require a considerable amount of upgrading, for example by cracking and by hydrotreating, in order to obtain more valuable hydrocarbon products. Upgrading by cracking, either thermal cracking, hydrocracking and/or catalytic cracking, is also effective to partially convert heavy hydrocarbon fractions such as atmospheric or vacuum residues derived from refining a crude oil or hydrocarbons derived from liquefying coal into lighter, more valuable hydrocarbons.
Numerous processes have been developed to crack and treat disadvantaged crude oils and heavy hydrocarbon fractions to recover lighter hydrocarbons and to reduce metals, sulfur, nitrogen, and acidity of the hydrocarbon-containing material. For example, a hydrocarbon-containing feedstock may be cracked and hydrotreated by passing the hydrocarbon-containing feedstock over a catalyst located in a fixed bed catalyst reactor in the presence of hydrogen at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Another commonly used method to crack and/or hydrotreat a hydrocarbon-containing feedstock is to disperse a catalyst in the feedstock and pass the feedstock and catalyst together with hydrogen through a slurry-bed, or fluid-bed, reactor operated at a temperature effective to crack heavy hydrocarbons in the feedstock and/or to reduce the sulfur content, nitrogen content, metals content, and/or the acidity of the feedstock. Examples of such slurry-bed or fluid-bed reactors include ebullating-bed reactors, plug-flow reactors, and bubble-column reactors.
Formation of high molecular weight sulfur containing heteratomic hydrocarbons, however, is a particular problem in processes for cracking a hydrocarbon-containing feedstock having a relatively large amount of heavy hydrocarbons such as residue and asphaltenes. Substantial amounts of high molecular weight sulfur-containing hydrocarbons are formed in the current processes for cracking heavy hydrocarbon-containing feedstocks. Such high molecular weight sulfur-containing heteroatomic hydrocarbons are difficult to remove from the resulting cracked product to produce a desirable low-sulfur hydrocarbon hydrocarbon product.
Cracking heavy hydrocarbons involves breaking bonds of the hydrocarbons, particularly carbon-carbon bonds, thereby forming two hydrocarbon radicals for each carbon-carbon bond that is cracked in a hydrocarbon molecule. Numerous reaction paths are available to the cracked hydrocarbon radicals, the most important being: 1) reaction with a hydrogen donor to form a stable hydrocarbon molecule that is smaller in terms of molecular weight than the original hydrocarbon from which it was derived; and 2) reaction with another hydrocarbon or another hydrocarbon radical to form a hydrocarbon molecule larger in terms of molecular weight than both the cracked hydrocarbon radical and the hydrocarbon with which it reacts—a process called annealation. The first reaction is desired, it produces hydrocarbons of lower molecular weight than the heavy hydrocarbons contained in the feedstock—and preferably produces naphtha, distillate, or gas oil hydrocarbons. The second reaction is undesired and leads to the formation of coke and the formation of high molecular weight sulfur-containing heteroatomic hydrocarbons as the reactive hydrocarbon radical (potentially containing sulfur) combines with another hydrocarbon (potentially containing sulfur) or hydrocarbon radical (potentially containing sulfur). Furthermore, the second reaction is autocatalytic since the cracked hydrocarbon radicals are reactive with the growing sulfur-containing hydrocarbons.
Hydrocarbon-containing feedstocks having a relatively high concentration of heavy hydrocarbon molecules therein are particularly susceptible to the formation of high molecular weight sulfur-containing hydrocarbons due to the presence of a large quantity of high molecular weight sulfur-containing hydrocarbons in the feedstock with which cracked hydrocarbon radicals may combine to form higher molecular weight sulfur-containing hydrocarbons. As a result, conventional cracking processes of heavy hydrocarbon-containing feedstocks tend to produce significant quantities of high molecular weight sulfur-containing hydrocarbons which render desulfurization of the resulting product difficult due to the refractory nature of such high molecular weight sulfur-containing hydrocarbons.
Conventional hydrocracking catalysts utilize an active hydrogenation metal, for example a Group VIII metal such as nickel, on a support having Lewis acid properties, for example, silica, alumina-silica, or alumina supports. It is believed that cracking heavy hydrocarbons in the presence of an acid or a material with acidic properties results in the formation of cracked hydrocarbon radical cations. Hydrocarbon radical cations are most stable when present on a tertiary carbon atom, therefore, cracking may be energetically directed to the formation of tertiary hydrocarbon radical cations, or, most likely, a cracked hydrocarbon may rearrange to form the more energetically favored tertiary radical cation. Hydrocarbon radical cations are unstable, and may react rapidly with other hydrocarbons.
Should a tertiary radical cation react with another hydrocarbon to form a larger hydrocarbon, the reaction may result in the formation of a carbon-carbon bond that is not susceptible to being cracked again. When either the cracked hydrocarbon radical cation or a hydrocarbon that reacts with the hydrocarbon radical cation contains sulfur, a sulfur-containing hydrocarbon compound having a higher molecular weight than either the hydrocarbon radical cation or the hydrocarbon with which the hydrocarbon radical cation reacts is formed. As a result, cracking utilizing conventional acid-based cracking catalysts produces significant quantities of refractory high molecular weight sulfur-containing hydrocarbon compounds.
Improved hydrocarbon compositions containing significant quantities of non-refractory relatively low molecular weight sulfur-containing hydrocarbon compounds that may be easily desulfurized that may be derived from cracking heavy hydrocarbon-containing feedstocks are desirable.